1. Field of the Invention
This invention relates generally to a cellulosic coating particularly useful in combination with osmotic controlled drug delivery devices. Specifically, a cellulosic coating is combined with a pore forming agent, preferably urea, to form a microporous coating used in conjunction with an osmotically controlled drug delivery device.
2. The Prior Art
U.S. Pat. No. 4,330,338 ('338 Patent) describes pharmaceutical coatings in the form of aqueous dispersions of a water insoluble polymer, such as ethyl cellulose. In the '338 Patent, the aqueous dispersions of water-insoluble polymer are prepared by dissolving the polymer in an organic solvent more volatile than water, emulsifying the solution thus formed in a continuous water phase, and thereafter removing the organic solvent to form the latex coating. The ethyl cellulose dispersion and other coatings described in the '338 Patent provide an alternative to organic, solvent-based tablet coating formulations which suffer from environmental, safety and toxicity problems.
Tablet cores coated according to the '338 Patent release active agent by diffusion, which can be an exceedingly slow process unless systems with a large surface area are utilized. Diffusive coatings, such as those described in the '338 Patent, therefore, are most often used to coat high surface area nonpariels or beads. To expedite release of active agent from tablets coated with, e.g., an ethyl cellulose coating prepared in accordance with the disclosure of the '338 Patent, hydroxypropylmethycellulose (HPMC) has been added to the coating. HPMC, a water soluble polymer, enhances the water solubility of the ethyl cellulose coats, causing the coating and subsequently the tablet core to rapidly disintegrate in an environment of use such as the intestine. Such rapid disintegration, however, is unsuitable for those applications in which drug is to be delivered continuously over time.
U.S. Pat. No. 4,060,598 ('598 Patent) describes the use of water-soluble or alkaline-soluble materials in combination with tablet coatings made from aqueous dispersions of synthetic polymers such as polyvinyl esters, polyacrylic acid esters, polyvinyl acetals, polyvinyl chloride or butadienestyrene copolymers to ensure that the synthetic resin coating (upon exposure to an aqueous environment of use) becomes sufficiently porous to enable active agent to diffuse through the coating.
The need exists, therefore, for an organic-free tablet coating suitable for use with osmotically-controlled drug delivery devices. Ideally, this coating, upon exposure to an aqueous environment of use, would transform to a microporous state and ensure continuous release of active agent from a tablet core to that environment.